We are studying the metabolism and enzymes of 2,3-diphosphoglycerate (2,3,-DPG) in the human red blood cell. It is known that the level of 2,3-DPG in red cells in vivo or in vitro (e.g., stored red cells) changes in response to pH and hypoxia but the controlling mechanisms are not understood. In various anemias the level of 2,3-DPG is abnormally high. The 2,3-DPG phosphatase of red cells is activated by a variety of anions and we are trying to define the physiologically important ones. We have recently completed a 31P-nuclear magnetic resonance study of normal human red cells and have calculated the level of ionized magnesium in the cells. The results indicate that the anaerobic level is three times the aerobic level. The data enable us to calculate the kinetically significant levels of metabolites that are involved in Mg2 ion-dependent reactions and to understand the effects of changing oxygen tension on metabolic rates in the red cells. We are studying the kinetics of phosphoglycerate mutase. Anions have complex effects on the kinetic parameters and we have defined these to some extent for simple salts like chlorides and acetates. We are continuing to study the effects of more complex ionic compounds such as citrate. We are planning to do nuclear magnetic resonance studies observing protons and phosphorus in the hope of defining the interactions more clearly.